1. Field of the Invention
The present invention relates to an ion implanting method, and more particularly to an ion implanting method which can remarkably reduce the fluctuations of an impurity concentration to be implanted in the depthwise direction.
2. Description of the Prior Art
In a magnetic bubble memory element, as is well known in the art, a bias field is applied to a magnetic film, which has a uniaxial magnetic anisotropy, such as a magnetic garnet film thereby to form magnetic bubbles, by which the memory action is effected.
In order to increase the memory density of the magnetic bubble memory element, recently, the diameter of the magnetic bubbles is remarkably reduced, and the practice of an element using minute bubbles having a diameter equal to or smaller than 2 .mu.m is earnestly promoted.
If, the diameter of the magnetic bubbles is reduced, however, it is quite natural that the propagation patterns to be used for their propagation will become remarkably minute. The precise formation of such minute propagation pattern is considerably difficult, raising the maximum difficulty in realizing the element having the minute bubbles.
More specifically, the propagation pattern of the magnetic bubble memory element generally uses the so-called "chevron or TI pattern", which is formed by photo-lithography.
The minimum line width that can be precisely formed by the photo-lithography has its limit at about 2 .mu.m, thus making it difficult to form such a fine pattern precisely with excellent reproductivity as has a smaller line width than 2 .mu.m.
Especially the clearance between adjacent patterns (which will be shortly referred to as "the gap") has to be made smaller than the aforementioned line width. The existing photo-lithography has failed to precisely form such minute gap, thus making it remarkably difficult to make the magnetic bubbles minute.
In order to solve the problems thus far described thereby to make the propagation of the minute bubbles possible, there has been proposed a CD element which uses neither the conventional chevron nor TI pattern but a propagation pattern having no gap.
The CD element is characterized, as diagrammatically shown in FIG. 1, in that a magnetic bubble propagation circuit 1 is of a shape having discs chained (and is called "the contiguous disc pattern", which is shortly referred to as "the CD pattern") and in that there is no gap in the propagation circuit so that magnetic bubbles 2 are propagated along the outer edge portion of the propagation circuit 1.
As shown in FIGS. 1 and 2, more specifically, a chain-shaped mask 1' made of a photoresist or an alloy of gold or aluminum-copper is formed on a magnetic film 4 which is so formed on a non-magnetic substrate 3, while having a uniaxial anisotropy, that it can hold the magnetic bubbles, and hydrogen, helium or other ions 5 are implanted into the remaining exposed portion thereby to form an ion-implated region 6. The portion covered with the mask 1' has no ion implanted area to provide such a propagation circuit as is shown in FIG. 1.
In this instance, the kind of the ions implanted and the voltage for the implantation are so selected that the ions reach one third depth from the surface of the magnetic film 4 thereby to establish distortion in the crystal lattice of the magnetic film 4.
For example, in case a film of (YSmLuCa).sub.3 (GeFe).sub.5 O.sub.12 having a thickness of 1.5 .mu.m is used as the aforementioned magnetic film 4, it is sufficient to implant the He ions at a voltage of 150 KeV.
Since the bubble (or magnetic) film for the CD element is generally so selected as to have a negative magnetostriction constant, the magnetization will lie parallel to the film plane due to the adverse effect of the magnetostriction if the crystal is subjected to the compressive stress as a result of the ion implantation. As shown in FIG. 2, consequently, the direction 7 of magnetization of the surface region 6, which is not covered with the chain-shaped mask 1' so that it is implanted with the ions, lies parallel to the film plane so that "the magnetic wall having magnetic charges" (which is usually called "the charged wall") is formed in the boundary of the CD pattern by the application of inplane magnetic field.
The bubbles 2 existing in the boundary are either attracted or repulsed by the charged wall so that they can be propagated by applying a revolving magnetic field from the outside thereby to shift the position of the charged wall.
As is apparent from the foregoing description, the CD element has to be formed with the charged wall so as to propagate the bubbles. For this purpose, the inplane magnetization has to be established in the vicinity of the surface of the bubble films.
Generally speaking, however, as the bubbles are made the more minute, the uniaxial anisotropy energy Ku (.ident.1/2MsHk, wherein Hk indicates the anisotropy field) required for the bubbles to stably exist is increased the more.
In order to establish the inplane magnetization required for providing the charged wall, therefore, the magnetostriction anisotropy energy Ki (.ident.3/2.lambda..sub.s .sigma..sub.s, wherein .lambda..sub.s indicates an inplane magnetostriction constant; and .sigma..sub.s indicates an inplane stress.) required for overcoming the increased Ku is increased.
Of these, however, the inplane magnetostriction constant .lambda..sub.s is determined by the kind of the bubble magnetic films, and the remaining quantity having a high degree of freedom is limited to the inplane stress .sigma..sub.s, the limit of which is the elastic limit of the crystal lattice.
In case a single kind of ions are implanted with a single energy into the bubble films, generally speaking, the distortion established in the films as a result of the ion implantation is distributed substantially in accordance with the Gaussian distribution about the depth R.sub.p, which is determined by the kind of the ions and the voltage for the implantation, while following the shape of a curve a indicated in broken line in FIG. 3. In this Figure: the abscissa indicates the depth of the ions implanted from the surface; and ordinate indicates a relative etching rate. Since the etching rate is increased the more for the larger distortion, the ordinate corresponds to the amplitude of the distortion.
As is apparent from the curve a of FIG. 3, the ions cannot be implanted uniformly in the depthwise direction, if the single ions are implanted with a single energy as by the ion implanting method in current use according to the prior art. As a result, the distribution of the distortion in the depthwise direction cannot be made uniform thereby to make it difficult to establish the uniform distribution of distortion required for the CD element.